your nose.
The next acts take place in your mouth, your stomach, and your small and large intestines.
Your brain, eyes, and nose
When you see appetizing food, you experience a conditioned response. (For the lowdown on how your digestive system can be conditioned to respond to food, see Chapter 14; for information on your food preferences, see Chapter 15.) In other words, your thoughts — “Wow! That looks good!” — stimulate your brain to tell your digestive organs to get ready for action.
What happens in your nose is purely physical. The tantalizing aroma of good food is transmitted by molecules that fly from the surface of the food to settle on the membrane lining of your nostrils; these molecules stimulate the receptor cells on the olfactory nerve fibers that stretch from your nose back to your brain. When the receptor cells communicate with your brain, your brain sends encouraging messages to your mouth and digestive tract as the sight and scent of food make your mouth water and your stomach contract in anticipatory hunger pangs.
What if you hate what you see or smell? For some people, even the thought of liver is enough to make them want to leave the room. At that point, your body takes up arms to protect you: You experience a rejection reaction. Your mouth purses, and your nose wrinkles as if to keep the food (and its odor) as far away as possible. Your throat tightens, and your stomach turns as muscles contract, not in anticipatory pangs but in movements preparatory for vomiting up the unwanted food. Not a pleasant moment.
But assume that you like what’s on your plate. Go ahead. Take a bite.
Your mouth
Lift your fork to your mouth, and your teeth and salivary glands swing into action. Your teeth chew, grinding and breaking food into small, manageable pieces. As a result,
You can swallow easily.
You break down the indigestible wrapper of fibers surrounding the edible parts of some foods (fruits, vegetables, whole grains) so that your digestive enzymes can get to the nutrients inside.
At the same time, salivary glands under your tongue and in the back of your mouth secrete the watery liquid called saliva, which performs two important functions:
It moistens and compacts food so your tongue can push it to the back of your mouth and you can swallow, sending the food down your esophagus into your stomach.
It provides amylases, enzymes that start the digestion of complex carbohydrates (starches), breaking the starch molecules into simple sugars. (Check out Chapter 8 for more on carbs.)
No protein digestion occurs in your mouth, and although saliva does contain very small amounts of lingual lipases — fat-busting enzymes secreted by cells at the base of the tongue — the amount is so small that the fat digestion here is insignificant.
TURNING STARCHES INTO SUGARS
Salivary enzymes (like amylases) don’t lay a finger on proteins and leave fats pretty much alone, but they do begin to digest complex carbohydrates, breaking the long, chainlike molecules of starches into individual units of sugars. The following simple experiment enables you to taste firsthand the effects of amylases on carbohydrates.
1 Put a small piece of plain, unsalted cracker on your tongue.No cheese, no chopped liver — just the cracker, please.
2 Close your mouth and let the cracker sit on your tongue for a few minutes.Do you taste a sudden, slight sweetness? That’s the salivary enzymes breaking a long, complex starch molecule into its component parts (sugars).
3 Now swallow.The rest of the digestion of the starch takes place farther down, in your small intestine.
Your stomach
If you were to lay your digestive tract out on a table, most of it would look like a simple, rather narrow, tube. The exception is your stomach, a pouchlike structure just below your esophagus, which few non-physicians have ever seen except for those TV viewers addicted to the show My 600-lb. Life.
Like most of the digestive tube, your stomach is circled with strong muscles whose rhythmic peristaltic contractions turn your stomach into a sort of food processor that mechanically breaks pieces of food into ever smaller particles. While this is going on, glands in the stomach wall are secreting stomach juices — a potent blend of enzymes, hydrochloric acid, and mucus.
One stomach enzyme — gastric alcohol dehydrogenase — digests small amounts of alcohol, an unusual nutrient that can be absorbed directly into your bloodstream even before it’s been digested. Other enzymes, plus stomach juices, begin the digestion of proteins and fats, separating them into their basic components, amino acids and fatty acids.
If the words amino acids and fatty acids are completely new to you and if you’re suddenly consumed by the desire to know more about them this instant, stick a pencil in the book to hold your place and flip to Chapters 6 and 7 for the details.
For the most part, digestion of carbohydrates comes to a temporary halt in the stomach. Stomach acids can break some carb bonds, but overall, the liquids here are so acidic that they deactivate amylases, the enzymes that break complex carbohydrates apart into simple sugars. Eventually, your churning stomach blends its contents into a thick soupy mass called chyme (from cheymos, the Greek word for “juice”). When a small amount of chyme spills past the stomach into the small intestine, the digestion of carbohydrates resumes in earnest, and your body begins to extract nutrients from food.
Your small intestine
Open your hand and put it flat against your belly button, with your thumb pointing up to your waist and your little finger pointing down.
Your hand is now covering most of the relatively small space into which your 20-foot-long small intestine is neatly coiled. When the partially digested chyme spills from your stomach into this part of the digestive tube, a whole new set of gastric juices are released:
Pancreatic and intestinal enzymes finish the digestion of proteins into amino acids.
Bile, a greenish liquid (made in the liver and stored in the gallbladder), enables fats to mix with water (emulsification like an oil and vinegar dressing).
Alkaline pancreatic juices make the chyme less acidic so that amylases can go back to work separating complex carbohydrates into simple sugars.
Intestinal alcohol dehydrogenase digests alcohol not previously absorbed into your bloodstream.
While these chemicals work, contractions of the small intestine continue to move the food mass down through the tube so your body can absorb sugars, amino acids, fatty acids, vitamins,
